Competition, seed dispersal and hunting: what drives germination and seedling survival in an Afrotropical forest?

Ola Olsson,Edu O Effiom,Ulf Ottosson,Henrik G Smith,Gabriela Nuñez-Iturri

doi:10.1093/aobpla/plz018

Ola Olsson, Edu O Effiom + Show 3 more

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https://doi.org/10.1093/aobpla/plz018

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Abstract

Disentangling the contributions of different processes that influence plant recruitment, such as competition and seed dispersal, is important given the increased human-mediated changes in tropical forest ecosystems. Previous studies have shown that seedling communities in an Afrotropical rainforest in southeastern Nigeria are strongly affected by the loss of important seed-dispersing primates, including Cross River gorillas (Gorilla gorilla diehli), chimpanzee (Pan troglodytes elioti) and drill (Mandrillus leucophaeus). Here we study how germination and survival of tree seedlings are affected by competition and reduced seed dispersal in three contiguous forest reserves, in southeastern Nigeria, with similar mature tree species composition and structure. We use an experimental design aimed at manipulating the effect of competition among seedlings in three protected and three hunted sites within the reserves. We use a total of sixty 5 × 5 m plots of three types: plots cleared of all seedlings, plots selectively cleared of all primate-dispersed seedlings and control plots. All seedlings were identified, measured, assigned to dispersal mode and tagged, and after 1 year we evaluated survival, mortality and new recruits. We found that in hunted sites germination of abiotically dispersed species was over four times higher in cleared plots compared to control plots, whereas germination of primate-dispersed species was the same, which indicated that dispersal limitation was the dominant force in seedling recruitment in hunted sites. This was supported by the fact that the germination of all dispersal modes in the selectively cleared plots in protected sites was similar to the control plots in the same sites, but germination of abiotically dispersed species was significantly lower than in cleared plots in hunted sites. Competition among seedlings was mostly evident from the fact that 75 % more seedlings of primate-dispersed species germinated in cleared compared to control plots in protected sites. We conclude that inter-seedling competition may be irrelevant to seedling recruitment in hunted sites, where dispersal limitation appears to be a much stronger force shaping the seedling plant community, and thus hunting indirectly reverses the importance of competition and dispersal limitation in structuring seedling communities.

Highlights

Hunting in otherwise intact tropical forests removes organisms from some trophic levels, which can change population and community dynamics at other levels (Dirzo and Miranda 1991; Terborgh et al 2001; Fa and Brown 2009; Lambert 2011; Effiom et al 2013, 2014)
Previous studies have shown that seedling communities in an Afrotropical rainforest in southeastern Nigeria are strongly affected by the loss of important seed-dispersing primates, including Cross River gorillas (Gorilla gorilla diehli), chimpanzee (Pan troglodytes elioti) and drill (Mandrillus leucophaeus)
In control plots, abiotically dispersed species were over three times as common in hunted sites compared to protected sites

Summary

Introduction

Hunting in otherwise intact tropical forests removes organisms from some trophic levels, which can change population and community dynamics at other levels (Dirzo and Miranda 1991; Terborgh et al 2001; Fa and Brown 2009; Lambert 2011; Effiom et al 2013, 2014). The protected sites have a more or less intact seed disperser community, with primates including the Cross River gorilla (Gorilla gorilla diehli), chimpanzee (Pan troglodytes elioti), drill (Mandrillus leucophaeus) and several species of smaller monkeys including putty nosed monkey (Cercopithecus nictitans) and Mona monkey (C. mona), whereas the hunted sites are practically empty of these animals (Effiom et al 2013, 2014) The seed predators, such as large rodents, are more common in hunted sites (Effiom et al 2013, 2014). While the patterns are clear, the underlying ecological processes are less well known (Stoner et al 2007; Vanthomme et al 2010; Effiom et al 2014; Rosin and Poulsen 2016)

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